1. Field of the Invention
The present invention relates to motor vehicle standard transmission clutches and more particularly to an air assist system for reducing clutch pedal effort in a standard transmission equipped motor vehicle.
2. Description of the Problem
Both standard and automatic shift transmissions are well known in the motor vehicle industry. Standard transmissions, sometimes called manual transmissions, remain common on light cars, trucks and busses, due to their mechanical simplicity, lower initial cost, lower weight and perceived efficiency relative to automatic transmissions. Standard transmissions typically provide a shift lever which is hand operated by the driver of a vehicle to move the transmission between the various available gear ratios provided in the transmission. The shift lever is connected with a linkage system to selectively engage a selected gear of an input shaft/counter-shaft standard transmission. During the disengagement of one gear ratio and the subsequent engagement of another gear ratio, a clutch is used to mechanically disengage the transmission from the vehicle""s engine. Disengagement of the clutch is typically done by the driver depressing a clutch pedal located under the instrument dash board and against the vehicle dash panel. The clutch pedal is connected to a clutch lever using a clutch linkage.
Standard transmissions installed on trucks can require excessive clutch effort on the part of some drivers or in stop and go situations. As a result, the driver can become tired or uncomfortable. The value in providing a mechanism to reduce the effort required to disengage clutches has long been recognized. U.S. Pat. No. 4,717,002 to Sasamura teaches one such clutch linkage power assist mechanism. Sasamura relies on pressurized air available on heavy duty trucks to drive a piston/cylinder arrangement which is connected to the linkage to provide a boost to the driver""s leg pressure to disengage a clutch. The cylinder receives air from and discharges air through a control valve. Valve operation is controlled by being directly connected into the clutch linkage. Sasamura also describes prior art clutch linkage systems using a piston/cylinder boost arrangement to reduce driver clutch pedal effort. He describes such systems as having to time properly the assistance provided by the air cylinder by sensing the build up of force on the clutch pedal to provide air to the cylinder in direct proportion to the clutch pedal force. Such sensitivity to clutch pedal force is obtained by incorporating the valve into the clutch linkage, a feature which Sasamura retained. Splicing the valve into the linkage was seen as assuring the capability of operating the clutch in a purely manual mode without boost, should the vehicle air system be compromised or unavailable.
An object of the present invention is to provide a clutch linkage requiring reduced effort for disengaging a standard transmission clutch.
Another object of the present invention is allow operator selection of the clutch effort required for disengaging a clutch.
Still another object of the present invention is to provide clutch linkage operation which is relatively immune to failure of the clutch disengaging boost system.
The invention provides for these and other objects in a standard transmission for a vehicle. The transmission comprises a clutch having an engaged state and a disengaged state. The clutch is moved between engaged and disengaged states using a clutch pedal which is connected to the clutch by a linkage assembly. Depression of the clutch from a default position moves the linkage assembly which in turn moves the clutch from its engaged state to its disengaged state. A spring provides for return of the linkage assembly to the default position, which returns the clutch to its default engaged state.
A powered actuator is coupled to the linkage assembly for generating a force urging the linkage assembly in the direction disengaging the clutch. Where the actuator is powered by compressed air, a relief valve operates to vent air from the powered actuator when open and to prevent venting when closed. The relief valve state is controlled by the position of the clutch pedal in a free travel. When the clutch pedal is in a first position in the free travel the valve is open and upon movement of the clutch pedal from the first position in the free travel the valve is closed. For compressed air systems the pressure assist is operator selected. An energization valve operates to deliver compressed air from the source of compressed air to the powered actuator when open, the energization valve being responsive to movement of the clutch pedal to a second position in the free travel.
The first and second points in the free travel of the clutch pedal are associated with first and second switches which are actuated by movement of the clutch pedal. First and second solenoid valve actuators provide for operating the relief valve and the energization valve, respectively, in response to changes in state of the first and second switches. The valves are arranged such that the energization valve has an inlet for receiving air from the source of compressed air, an exhaust port and an outlet to the compressed air powered actuator, the energization valve being operable in an open state to connect the inlet to the outlet and in the closed state for connecting the outlet to the exhaust port. The relief valve has an inlet connected to the exhaust port and an outlet, and being operable in a closed state to block communication between its inlet and its outlet and in an open state to connect its inlet to the outlet.
Additional effects, features and advantages will be apparent in the written description that follows.